A car bomb ripped through a busy commercial district in a rebel-held Syrian town along the Turkish border Saturday, killing nearly 50 in a huge explosion that damaged buildings and left rescuers scrambling to find survivors amid the wreckage, opposition activists said.

Rescuers and doctors said the explosion was so large there were nearly 100 wounded and burned. Over 50 wounded were transported to the Turkish border town of Kilis for treatment, as local hospitals couldn't cope.

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There was no immediate claim of responsibility. Locals said a rigged tanker caused the explosion and blamed Islamic State militants, who have carried out attacks in the town before. The militant group has been increasingly pressed in Syria and Iraq, and has escalated its attacks against Turkey — which backs Syrian opposition fighters in a campaign against the group in northern Syria.

Azaz, only a couple of miles from the Turkish border, is a key town on a route used by opposition fighters moving between Syria and Turkey, and is a hub for anti-government activists as well as many displaced from the recent fighting in Aleppo city. Activists say its pre-war population of 30,000 has swelled.

It is also sandwiched between rival groups, including Kurdish fighters to the west and Turkey-backed opposition groups to the east. Islamic State militants, who have tried to advance on the key border town before have been pushed back farther east in recent months in the Turkey-backed offensive.

The bomb went off early Saturday afternoon outside a local courthouse and security headquarters operated by the opposition fighters who control the town, resident and activist Saif Alnajdi told The Associated Press from Azaz.

"It hit the busiest part of the town," Alnajdi said, referring to the administrative part of town.

A medical worker speaking to a local media outfit, al-Jisr, said many charred bodies, and body parts mixed with bones and mud, were piled up in local hospitals.

Rami Abdurrahman, the head of the Syrian Observatory for Human Rights monitoring group, said at least 48 people were killed, including 14 fighters and guards to the local courthouse. He said the explosion was caused by a rigged water or fuel tanker, which explained the large blast and high death toll. The activist-operated local Azaz Media center and Shabha Press put the death toll at 60, adding that search and rescue operations continued for hours after the explosion.

Alnajdi said rescue workers were still working to identify and remove the bodies from the area, suggesting that the death toll was not final. He said some of the severely wounded were transported across the border into the Turkish town of Kilis for treatment. The Turkish state-run Anadolu news agency said 53 wounded Syrians were brought to Kilis' local hospital for treatment, including five in critical condition, transferred to Gaziantep. The agency said one later died.

Media activist Baha al-Halabi, based in Aleppo province and who gathered information from Azaz residents, said witnesses reported many unidentified bodies. Footage shared online showed a large plume of black smoke rising above the chaotic street with the sound of gunfire in the background as onlookers gathered around the site. In one instance, a father ran away from the scene, carrying his child to safety.

The court house and the security headquarters were damaged, as well as the Red Crescent and municipality offices, according to activists in the area.

Many rebels and civilians who were pushed out of Aleppo city during a massive government offensive late last year have resettled in Azaz. Syrian Kurdish forces control territory to the west of Azaz, and have often tried advancing toward the town, causing friction with Turkish troops and allied Syrian opposition fighters. To the east, opposition fighters backed by Turkey have been pushing back Islamic State extremists, gaining territory and advancing on the IS-stronghold town of al-Bab, further east. Turkey considers Syria Kurdish factions there terrorists, linked to a local group it is battling at home.

A nationwide week-long cease-fire has mostly held across most of Syria after Russia and Turkey, who support opposite sides of the conflict, reached an agreement late December. It is set to pave the way for peace talks between Assad's government and the opposition in Kazakhstan later this month. The Islamic State group and al-Qaida-linked group Fatah al-Sham Front are not included in the deal, according to the Syrian government.